In the fabricating of integrated circuitry, different elevation conductive and/or semiconductive layers are commonly separated by insulative layers. Electrical connections are commonly made between different elevation devices by forming contact openings through an insulating layer prior to forming the higher elevation devices. Such openings are typically formed through a masking layer using an anisotropic plasma etching technique. One particular class of tools for doing so is known as a bottom powered, dual frequency etch system. It has been found in certain instances when operating such systems in the manner recommended by the manufacturer that electrical characteristics of the fabricated circuitry were being shifted or changed in undesirable manners. One discovered adverse effect was a decrease in the field threshold voltage. This is a measurement of the voltage required to form undesired parasitic field effect transistors across field isolation on the substrate. It is typically desirable that this voltage be as high as possible to avoid the formation of such parasitic devices.
An exemplary plasma etch apparatus and data from semiconductors processed according to the prior art are shown in FIGS. 1-3. Referring to FIG. 1, an exemplary bottom powered dual frequency plasma etch apparatus is shown. Apparatus 10 includes a bottom powered electrode 12 and a top unpowered, grounded electrode 14. Electrode 12 has dual frequency power sources of 2 and 27 MHz respectively. An exemplary apparatus 10 is the Exelan system produced by LAM Research Corporation of Freemont, Calif., which is a bottom powered dual frequency etch system. During plasma etch, this system is powered simultaneously at 2 MHz and 27 MHz with substantially balanced or equal powers. For example, these powers are typically run at greater than 1000 watts each, with a power ratio from 0.6 to 1.67 of the 2 MHz frequency to the 27 MHz frequency (2 MHz:27 MHz).
Semiconductor substrates processed under these conditions can develop low field threshold voltages, as well as other adverse effects such as inadequate refresh times. FIGS. 2 and 3 illustrate the low field threshold values of semiconductor substrates processed in accordance with these prior art methods. Referring to FIG. 2, at high total power (3050 watts) and low frequency ratio of 0.65, the field threshold voltage was 5.92. Likewise, as illustrated in FIG. 3, at a total power of approximately 3050 watts and a high frequency ratio of about 1.6, the field threshold voltage was 5.52. It is preferred to maintain the total power as high as possible to increase etch rates, thereby processing substrates in a more rapid manner.
While the invention was motivated by addressing the above issues and challenges, it is, of course, in no way so limited. This invention is only limited by the accompanying claims as literally worded and appropriately interpreted in accordance with the doctrine of equivalents.